We are examining the role of thrombospondin-1 (TSP1) in regulating tumor growth, angiogenesis, and metastasis. These studies examine the direct effects of TSP1 expression on tumor cells, its role in neovascularization of tumors, and its role in tumor cell interactions with endothelium and the host immune response during metastasis. TSP1 is a matricellular protein that both stimulates and inhibits specific cellular responses. Some of the diversity in cellular responses to TSP1 may be explained by differential expression of several TSP1 receptors that induce distinct signals. However, we have recently found that a single TSP1 receptor, the a3b1 integrin, can mediate cell type-specific responses to TSP1 through differential regulation of its activation state. The a3b1 integrin is the major mediator of breast carcinoma cell adhesion and chemotaxis to TSP1. This integrin is constitutively expressed, but it is normally inactive in these cells. Signaling through G proteins, IGF1, and CD98 specifically activate the a3b1 integrin in breast carcinoma cells. These stimuli induce a3b1 integrin-mediated formation of filopodia and breast carcinoma cell chemotaxis to TSP. In small cell lung carcinoma (SCLC), the a3b1 integrin mediates adhesion and neurite outgrowth on immobilized TSP1. In contrast to breast carcinoma cells, EGF but not IGF1 stimulates function of a3b1 integrin in SCLC cells. EGF and TSP1 also synergize to inhibit proliferation of SCLC cells. The a3b1 integrin is also expressed in endothelial cells, but its activity to recognize TSP1 is suppressed in confluent cells. Loss of cell-cell contact or VE-cadherin signaling specifically activates the a3b1 integrin and mediates positive proliferative and angiogenic responses to TSP1. Therefore, these three cell types differentially regulate the activity of a single TSP1 receptor in response to discrete signals from their environment. These studies have provided new insights into the mechanisms by which TSP1 expression inhibits tumor growth and neovascularization. Using synthetic peptides and recombinant fragments, we have identified functional sites in thrombospondin-1 that express these activities. Peptide sequences containing the motif Trp-Ser-Xaa-Trp regulate endothelial cell motility and proliferative responses to basic fibroblast growth factor, inhibit proliferation and activate avb3 integrin in melanoma cells, and induce programmed cell death in endothelial cells but not in breast tumor cells. A peptide sequence from the amino-terminal pentraxin domain interacts specifically with a3b1 integrin and modulates tumor cell spreading, motility, and proliferation. Stable analogs of these peptides were prepared using D-reverse peptides and inhibit tumor growth and angiogenesis in animal models. These stable peptide analogs have potential clinical applications in cancer and other diseases associated with abnormal angiogenesis.